1. Field of the Invention
This invention relates to metal detectors of the type used to detect metal fragments in products, such as foodstuffs and pharmaceuticals, and is directed more particularly to a method for making an improved search coil frame assembly portion of such a detector.
2. Description of the Prior Art
In many industries, products destined for public consumption are conveyed automatically from station to station for processing. It is critical in some industries, as for example in the production of foodstuffs and pharmaceuticals, that tramp metal fragments introduced into the material processing stream be detected before the product is shipped to market.
It is known in the art that non-metallic material may be passed through an electromagnetic field and that if metal is present in the material such metal will disturb the electromagnetic field. A search coil is commonly used to create the electromagnetic field and metal passing through the field induces a signal indicating that the field has been disturbed. Responsive to the signal, the material stream can be stopped or diverted, accompanied if desired, by visual and/or audible alarm means.
The search coil assemblies of known metal detectors include a frame of non-metallic material, such as wood or plastic, having an aperture therethrough. The aperture is of size and configuration to permit passage therethrough of the product to be tested. For acceptable performance, it is necessary that the aperture be only slightly larger than the product passing therethrough. This requirement necessitates virtual custom tailoring for each product for which a machine is ordered, or maintenance of large inventory of various sizes by the manufacturer.
The frame portion of the assembly generally includes a top member, a bottom member, and two side members joined together to define a rectangular aperture. A known search coil assembly includes the frame, a carbon coating disposed over the external surfaces of the frame, a plastic coating disposed over the external surfaces of the carbon coating, coils of electrically conductive strands wound on the external walls of the plastic coating, a body of plastic adhered to the plastic coating and having the coils embedded therein, and a metal housing enveloping the plastic body. The electrically conductive strands typically include three strands, a "transmit" strand disposed centrally of the frame, and on either side thereof, and parallel thereto, a "receive" strand. Each of the strands has two free ends which pass through the plastic body and are connected to electronic circuitry disposed in the assembly or externally thereof.
A search coil assembly of the type described immediately above is shown and described in U.S. Pat. No. 4,814,734, issued Mar. 21, 1989, in the name of James M. Moran, and a method for making such assembly is shown and described in U.S. Pat. No. 4,819,321, issued Apr. 11, 1989, in the name of James M. Moran.
In an alternative construction, the electrically conductive strands are wound directly on the frame and enveloped in a coating of plastic applied to the external surfaces of the frame. A metal coating is, in turn, applied to the exterior surfaces of the plastic coating. On the internal walls of the frame there may be disposed a graphite coating, which is covered with a plastic sheeting. A search coil assembly of such construction is shown and described in U.S. Pat. No. 4,870,381, issued Sep. 26, 1989, in the name of James M. Moran, and a method for making such assembly is shown and described in U.S. Pat. No. 4,833,773, issued May 30, 1989, in the name of James M. Moran.
The outer metal layers of the assembly constructions described above may be covered with still another coating of plastic to protect the metal housing from scratches and other disfigurations, as shown and described in U.S. Pat. No. 4,902,997, issued Feb. 20, 1990, in the name of James M. Moran and U.S. Pat. No. 4,903,395, issued Feb. 27, 1990, in the name of James M. Moran.
The constructions described above have represented meaningful steps forward in the continuing search for methods of producing search coil assemblies at reduced expense and having equal or improved performance. A necessary portion of the search coil assembly is the frame portion, typically constructed of four blocks of wood or plastic, joined together to form the aperture, and the electrically conductive strands wound upon and/or around the frame. The process of properly joining the components, accurately winding the strands thereon, and providing a plastic body embedding the strands, involves considerable expenditure of time, and further requires a high degree of skill to avoid having to modify or rebuild the frame and coil assembly.
In U.S. patent application Ser. No. 07/557,626, filed Jun. 24, 1990, in the name of James M. Moran, there is shown and described a new construction of frame, involving laminate members joined together to form each frame member. In each laminate member is a layer of conductive material, such that upon joining of the frame members to form the frame assembly there is no need to add the usual graphite coating to provide shielding between the conductive strands and the aperture. In the '626 method of frame construction, the step of adding the conductive shield layer after completion of the frame is eliminated, but there remains the step of winding the conductive strands around the completed frame.
In order to further reduce the cost of manufacture of metal detectors, and to solve the ever-present problem of inventory and tailoring detectors to particular sizes and shapes of products, it would be beneficial to provide frame assemblies which are more simply and economically constructed and in which portions of the frame assembly may be inventoried and used for a number of sizes and shapes of apertures.